The present invention, a magnetic tape tension sensor, relates to the field of sensing devices, and in particular to the field of tape position and sensing devices as used in magnetic tape drive units.
In modern magnetic tape transport systems, the ability to sense the presence of tape tension during reel-to-reel operation is necessary to prevent uneven winding and damage to the tape media. In the prior art, it is well known to use mechanical switching devices to sense the tape tension, using the signal generated to operate the reels. Because of the varying position of the recording media, design parameters have acquired a relatively low force to operate the sensing mechanism which comes in contact with the tape media. This problem is compounded by the fact that the response of mechanical switches is non-linear with the operating force three to four times that of the release force, such that at the low tension levels necessary to prevent tape damage, the force generated by the tape tension causing the switch to be activated may be so low that the internal friction of the tension sensor assembly itself may be larger than the return force generated by the switch, thus causing the switch to remain in a closed position. The disclosed invention overcomes this problem by using an light switch having an arm which is pulled by gravity into an open position when not in contact with tape in a tensioned condition. In the open position, the light from the LED is twice reflected completing the light path back to the optical switch providing a signal to the appropriate circuit. When the tape is in a tensioned condition, the tape rotates the arm into a position such that the light beam is deflected causing the light path to be broken. However, when the tape is in an untensioned condition, gravity pulls the arm downward, again completing the light path.
In the prior art, it is known to use the interuption of a optical beam as a sensing device. See e.g., U.S. Pat. No. 3,800,162, dated Mar. 26, 1974, issued to Lueck et al, wherein an apparatus is disclosed for sensing the tension in a plurality of filaments. In Lueck, an unbalanced mass sufficient to deflect the filament a given amount is suspended between two independent light sources and detectors. Should the tension in the filament be greater or less than the specified amount, the detector arm swings into the light path, interupting same. One disadvantage with this prior art, is that the detector arm must remain in constant contact with the filament. The disclosed invention overcomes this problem by only contacting the tape when the tape is in a tensioned condition, having a restaining means to prevent the arm from moving into the tape path when the tape drive unit is in an operating mode other than reel to reel. The disclosed invention, when placed in a tape drive unit having severe space limitations, can sense tape tension in less than 0.050 inches of deflection.
It is an object of this invention to provide a means for sensing the tape tension in a magnetic tape drive unit.
It is yet another object of this invention to provide a low force means for sensing the tape tension in a magnetic tape drive unit.
It is still yet another object of this invention to provide a means for holding the tape away from the edge of the magnetic head in a magnetic tape drive unit when said unit is operating in a reel-to-reel mode.